Offshore platforms are used to house workers and support production equipment at sites located in a large body of water. They are used in such applications as petroleum drilling and production, Ocean Thermal Energy Conversion (OTEC), and remote radar installations. Depending on the circumstances, an offshore platform might be fixed to the ocean floor, built on an artificial island, or floating at the surface of the body of water.
For deep-water applications, floating platforms, such as spars and semi-submersible platforms are typically used. Such floating platforms are subject to motion due to dynamic wave and wind forces. Platform motion can cause unacceptable stresses in riser and mooring lines and, in some cases, can curtail deck operations for extended periods of time.
Platform motion is particularly problematic for petroleum drilling and OTEC applications. In petroleum drilling applications, for example, platform motions must be kept small because many of these platforms comprise riser pipes that are rigidly fixed to the sea bed and attached to the platform. Typical OTEC systems normally include large diameter cold water pipes, which are suspended from their platforms and hang down to cold deep water regions. These cold water pipes can have lengths of 1000 meters or more. Excessive platform motion induces severe strains in these cold water pipes, which can lead to system failure.
In order to increase the reliability and lifetime of floating platform installations, a number of methods for reducing motion of a floating offshore platform have been developed in the prior art. These include:                i. Providing a small waterplane area to reduce the wave loadings at the free surface;        ii. providing a deep draft to establish the keel of the body to be below the area of the highest wave energy, and to achieve a low center of gravity;        iii. using vertically rigid moorings; and        iv. providing hydrodynamic optimization, such as using wave force cancellation between the columns and pontoons of a semi-submersible platform.        
These methods, however, increase the cost and complexity of the floating platforms.
A Spar platform is based upon a large-diameter, single or multiple vertical cylinder(s) that supports a deck above the surface of the water. About 90% of a typical spar structure is underwater. The cylinder is analogous to a deep-draft floating caisson, which is a hollow cylindrical structure similar to a very large buoy. A distinguishing feature of a spar is its deep-draft hull, which produces very favorable motion characteristics compared to many other floating concepts.
Due to its deep draft, a typical spar is deployed by floating it horizontally in a harbor or quay, towing it to a deployment site, and then upending it into a vertical orientation. Once oriented vertically, a derrick barge is used to lift a deck structure into place. This process is extremely expensive and time consuming.
Semi-submersible platforms are platforms configured with large buoyant pontoon structures that float below the water surface. Structural columns, attached to the pontoons pass through the water surface to support a platform deck at a significant height above the sea surface. Semi-submersible platforms can be anchored to the ocean floor or kept in position by attached thrusters.
The draft of some semi-submersible platforms can be transformed from a deep-draft to a shallow-draft by removing ballast water from its hull. A shallow-draft platform is analogous to a surface vessel and can be towed from a harbor or quay to its deployment location by a tugboat. Once at its intended location, ballast water is added to back into the hull to return the platform to a deep-draft configuration.
With its hull structure submerged at a deep draft, the semi-submersible platform is analogous to a spar and is less affected by wave loadings than a normal ship. Since a typical semi-submersible platform has a small water-plane area, however, it is sensitive to load changes on its deck. As a result, careful trimming is necessary to maintain platform stability.
In addition, a conventional spar or semi-submersible is designed to satisfy a single, rigid set of operational requirements expected throughout its operational lifetime. As a result, the amount of buoyancy and deck space made available for equipment and personnel are pre-determined based upon several factors: the environmental characteristics of its intended deployment location; the intended application of the platform; and its desired production capacity.
Once a conventional floating platform has been deployed at its deployment location, the flexibility of a floating platform is limited by the pre-determined design. In order to increase production capacity (e.g., increase drilling depth, modify the deck for additional equipment, add additional energy conversion equipment, change the configuration of equipment, etc.), the platform must be transported to a drydock, where the additional equipment and additional buoyancy (if necessary) can be conveniently added. In addition to the large expense such an operation incurs, the platform is also removed from service during the period of transportation and refit.